Re: [SQL] inserts/updates problem under stressing !

On Mon, 26 Jul 1999, Vadim Mikheev wrote:

> Date: Mon, 26 Jul 1999 10:43:00 +0800
> From: Vadim Mikheev <vadim@krs.ru>
> To: Tom Lane <tgl@sss.pgh.pa.us>
> Cc: Oleg Bartunov <oleg@sai.msu.su>, pgsql-hackers@postgreSQL.org,
>     pgsql-sql@postgreSQL.org
> Subject: Re: [SQL] inserts/updates problem under stressing !
> 
> (Sorry for incomplete prev message).
> 
> Tom Lane wrote:
> > 
> > I wonder whether this doesn't have a problem with concurrent access:
> > 
> > 1. Transaction A does 'Select count into cnt', gets (say) 200.
> > 2. Transaction B does 'Select count into cnt', gets 200.
> > 3. Transaction A writes 201 into hits record.
> > 4. Transaction B writes 201 into hits record.
> > 
> > and variants thereof.  (Even if A has already written 201, I don't think
> > B will see it until A has committed...)
> 
> You're right, Tom.
> 
> > I am not too clear on MVCC yet, but I think you need "SELECT FOR UPDATE"
> > or possibly an explicit lock on the hits table in order to avoid this
> > problem.  Vadim, any comments?
> 
> SELECT FOR UPDATE will not help: if there was no record for
> particular key then nothing will be locked and two records with
> the same key will be inserted.
> 
> Oleg, use LOCK IN SHARE ROW EXCLUSIVE MODE.

Thanks Vadim. Just tried this, but still I see a difference between
count hits (accumulated) from db and access_log. In my test these numbers are:
95 and 109. So I lost 14 hits ! And no errors !
In my handler I have now:

my $sth = $dbh->do("LOCK TABLE hits IN SHARE ROW EXCLUSIVE MODE");
my $sth = $dbh->do("SELECT acc_hits($1)") || die $dbh->errstr;

am I right ?

I created hits table as:
create table hits (  msg_id int4 not null primary key, count  int4 not null, first_access datetime default now(),
last_accessdatetime
 
);

and in error_log sometimes I see 
ERROR:  Cannot insert a duplicate key into a unique index
How this could be possible if I use 
LOCK TABLE hits IN SHARE ROW EXCLUSIVE MODE ?

Oleg

PS.
I remind my functions is:

CREATE FUNCTION "acc_hits" (int4) RETURNS int4 AS '
Declare   keyval      Alias For $1;   cnt         int4;   curtime     datetime;
Begin   curtime := ''now'';   Select count into cnt from hits where msg_id = keyval;   if Not Found then       cnt :=
1;      -- first_access inserted on default, last_access is NULL       Insert Into hits (msg_id,count) values (keyval,
cnt);  else       cnt := cnt + 1;       Update hits set count = cnt,last_access = curtime where msg_id = keyval   End
If;  return cnt;
 
End;
' LANGUAGE 'plpgsql';



> 
> Vadim
> 

_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83